The invention relates generally to video overlay systems for computers and more particularly to video overlay systems that do not receive pixel clock data from an underlying graphic source generator.
Existing computer systems facilitate overlaying of video information on top of graphic information on computer screens. To accomplish this, many computer systems have a separate underlying graphics source card and a separate video overlay source generator (card). The graphics source card produces graphic images on the computer screen. The separate video overlay source system is used to overlay video images on top of graphic images produced by the graphics card. It is important that the overlaid video image be placed precisely where desired by the user and within the active region of the computer screen.
Often times, the video overlay source system is a peripheral unit added to the computer system. Therefore, the graphics card pixel clock is not communicated to the video overlay source system. However, to properly align video overlay data onto underlying graphics, the video overlay source system needs to know the pixel clock to properly display and align the video overlay so that the size of the active region of the display screen and the position of the active region of the display screen are used to properly align the video overlay information.
Because video overlay source generators may not know where the active screen region limits are located due to the fact that the pixel clock information is on the graphics controller card and not communicated to the overlay source system, the overlay source system must somehow obtain this information. In conventional overlay source systems, the size and position of the active region of the screen is typically estimated based on interactive information entered by the user.
Conventional overlay systems typically estimate a pixel clock of the graphics controller card by having the user draw and align two boxes. The software first draws a box with the graphic controller card outlining the active region of the computer screen and subsequently draws a similar sized box using the overlay source system. The user then moves the video overlay box to try to align it with the box generated by the graphics controller card. Once approximately aligned, the overlay source then estimates the pixel clock. This calibration process is a cumbersome process for the user and can result in alignment errors if the user does not properly size the boxes during the process or position the boxes at the correct position during the calibration process. If not properly sized and positioned, the overlay source may overlay video in a non-active portion of the display screen. Moreover, increased temperatures of the system can cause thermal drifting of the active screen location. In conventional systems, the user is required to recognize this condition and repeat the same interactive calibration process to compensate for the thermal drift.
Therefore, there exists a need to have a more accurate and time saving system to facilitate proper alignment of video overlays. It would be desirable if such a system automatically calibrated itself to properly align overlaying video data on a computer screen without the need for user interaction. Moreover it would be desirable to have an automatic system that could accommodate for thermal drift without requiring a user to manually interact and attempt to provide the alignment information for the computer system.